As electric linear motion actuators which include an electric motor as a driving source, those disclosed in the below-identified Patent Document 1 and Patent Document 2 are conventionally known.
Each of the electric linear motion actuators disclosed in the above mentioned Patent Document 1 and 2 include: a rotary shaft driven by an electric motor; an outer ring member supported movably in the axial direction; and planetary rollers mounted between the rotary shaft and the outer ring member. When the rotary shaft is rotated, the planetary rollers are allowed to rotate about their respective center axes while revolving around the rotary shaft, due to the interfacial friction between the rotary shaft and the planetary rollers, and the outer ring member moves in the axial direction due to the engagement between helical grooves or circumferential grooves provided on the radially outer surfaces of the respective planetary rollers and a helical rib provided on the radially inner surface of the outer ring member.
Since the electric brake system including the electric linear motion actuator disclosed in Patent Document 1 or 2 only has a function as a service brake, in which a braking force is controlled corresponding to the brake pedal operation by a driver, the electric motor needs to be kept energized in order to maintain the braking force at the time of parking, thereby consuming a large amount of electricity.
In order to solve the above mentioned problem, the present inventors have proposed an electric linear motion actuator, in which the braking force can be maintained even when the energization to the electric motor is terminated, in the below-identified Patent Document 3.
The electric linear motion actuator disclosed in Patent Document 3 includes a reduction gear mechanism configured to reduce the speed of the rotation of a rotor shaft of the electric motor and to transmit the reduced rotation to the rotary shaft. A plurality of locking portions are provided in a side surface of one of a plurality of gears constituting the reduction gear mechanism, circumferentially spaced apart from each other. When a locking pin capable of being moved forward and backward with respect to the locking portions is moved forward by actuating a linear solenoid, the locking pin is brought into engagement with one of the locking portions, thereby locking the gears. This allows the braking force to be maintained even when the energization to the electric motor is terminated.
Therefore, by including the above mentioned electric linear motion actuator in an electric brake system, it is possible to lock one of the brake pads with the brake pads pressing the disk rotor at a predetermined pressing force, at the time of parking. At the same time, an electric linear motion actuator compact in size can be provided.